The present invention relates to a method for the pre-reduction of laterite fines in a bubbling fluidized bed reactor with reducing gases generated in situ by adding a reducing agent such as a carbonaceous material in the fluidized bed chamber, fluidizing the bed with a sub-stoichiometric quantity of oxygen, and maintaining a temperature sufficiently high to partially combust the reductant and generate a reducing atmosphere.
Laterite ores used as a starting material for the production of nickel from smelting operations typically contain a significant fraction of particles greater than 10 cm in diameter as well as a significant fraction of particles less than 45 xcexcm in diameter. Dealing with such a wide size distribution is a complicated and troublesome task, and in spite of major efforts in attempting to overcome the difficulties associated therewith, conventional technologies like rotary kilnxe2x80x94electric furnace processing, are still compromised by the naturally occurring fines in the ore.
Pre-reduction is used in the pyrometallurgical treatment of nickel laterite ores to reduce the load placed on the nickel smelting furnace. This pre-reduction operation is for the specific purpose of removing oxygen present in the ore to yield a separate nickel enriched metal phase after smelting. The operating conditions and the amount of reducing agent provided in the reactor will depend on the ferronickel grade required and the composition of the ore used as starting material. This can be accomplished upstream of the furnace while the ore is in the solid state, or alternately, directly in the furnace during smelting. Although pre-reduction represents an additional step in the nickel smelting operation, it is beneficial because it reduces the overall process energy cost, as well as the required furnace size, the required capacity of the furnace offgas system and the required power plant size.
Commercially, pre-reduction is typically in the range of 40 to 70% of the total reduction required to produce ferronickel. This usually includes reducing a substantial portion of the iron from the ferric state to the ferrous state, some of the nickel oxide to metallic nickel, and a minor amount of the ferrous iron to metallic iron.
In rotary kiln pre-reduction, reducing gases can be generated in situ with coal. However, this typically limits the extent of pre-reduction to about 40% unless extremely long residence times are utilized, or if the reduction is assisted with oil. In shaft furnace pre-reduction, oil is required as well as a dedicated external gasifier. The main constraint in using in situ gasification of coal is the carbon content of the product calcine. Currently, the carbon content of the calcine produced from existing commercial operations ranges from over 1% for oil fired shaft furnace and rotary kiln operations, to 3% for coal fired rotary kiln operations. Alternative technologies such as those developed for the direct production of iron by solid state reduction provides carbon levels from 3 to 10% in the product. The necessity for a low carbon content in the product calcine is determined by the carbon requirements of the downstream smelting furnace. If the carbon content of the calcine is too high, it will lead to excessive reduction in the smelting furnace, thus producing additional metallic iron and lowering the grade of the ferronickel product. The end result will be a ferronickel product that does not meet the product specification for nickel content. It is also well known that physical removal of carbon from the hot calcine to sufficiently low levels is not possible.
Traditionally, in fluidized bed pre-reduction processes, the reducing gas is generated externally from the fluidized bed reactor with the help of a dedicated gasifier. However, the concept of using the partial combustion of a carbonaceous material in a fluidized bed reactor for the in situ generation of reducing gases is also known, but little literature is available on specific experimental conditions and compound properties allowing optimal operation.
U.S. Pat. No. 5,445,667 is concerned with a process for the reduction of iron ore in solid phase in a fluidized bed reactor. An excess of carbon in the form of coke or coal is added in situ in addition to oxygen, and the temperature is maintained higher than 850xc2x0 C. It would appear that the conditions used, i.e., the CO/CO2 ratio are such that the product obtained contains mainly metallic iron or iron carbide. The particle size of the iron oxide is up to 1 mm, and the CO/CO2 ratio is between 2.3 and 4.0. There is a reference to the fact that the method of this patent could be used to treat other materials containing iron oxide, but this statement is not supported by any experimental evidence whatsoever. In addition, the carbon content of the final calcine is not specified, and the physical properties of the coke or coal are not mentioned, suggested or implied. In addition, this process is based on the partial conversion of metallic iron to iron carbide in a separate reactor to inhibit sticking. The carbon content of iron carbide is 6.7%. This process also requires substantial preheating of the fluidizing air, preferably above 1000xc2x0 C., or substitution of air with pure oxygen to enable sustaining the required operating temperature. Such a process cannot be applied to laterite containing materials because the extent of reduction is greater than that required for ferronickel production and an equivalent nickel carbide species does not exist.
U.S. Pat. No. 4,224,056 describes a process for the reduction of iron ore fines with a fluidized bed, wherein the ore is reduced simultaneously with the reducing gas production, i.e., generated in situ. The carbon-bearing particles are fluidized with the fluidizing gas to form a bed of carbon-bearing particles in the reactor. The iron ores can be in various forms, including dust. Iron appears to be present in the metallic form in the calcine. The separation of carbon from the reduced product is claimed to take place directly in the reactor as a result of the difference in particle densities. The efficiency of this separation is very questionable, since the literature contains many examples of the high degree of vertical mixing in a fluidized bed reactor. The patent also discusses the difficulty in maintaining the reactor temperature and suggests electrical heating and an external gasifier as means of achieving the system energy balance.
U.S. Pat. No. 4,070,181 discloses a method for the reduction of finely divided metal oxides like iron ores, the reduction being accomplished in a reactor with a large excess of carbonaceous material. In fact, the coal addition is approximately 50% that of the ore addition. Each component is supplied continuously in the reactor. The grain size of the iron oxide is lower than 1 mm and the grain size of the coal is lower than 3 mm. The carbonaceous material can also be a liquid, such as oil. The temperature is preferably maintained between 800 and 1100xc2x0 C. in the reactor. The method can also be applied to the reduction of nickel oxide. The patent further states that the pre-reduced product containing coke proceeds to a final reduction stage. This reference is the basis of the first step in the Elred process that has been described in the literature, but was never commercialized. In fact, a paper authored by the inventor of the invention disclosed and claimed in U.S. Pat. No. 4,070,181 states that the carbon content of the partially pre-reduced calcine contains about 26% carbon based on pilot plant tests (see Widdell et al. in Iron and Steelmaker, October 1981, pp. 219-224).
Pahlman et al. in Mining Review, October 1976, pp. 16-20, discloses a fluidized bed reduction roasting process applied to taconites, wherein carbonaceous fuels are partially combusted to supply heat and reductant requirements. The fact that the carbonaceous fuels are only partly combusted means that the resulting calcine will have a significant carbon content, i.e., around 6%, as calculated by the present inventors. The particle size of the carbonaceous material used is minus 8 plus 28 mesh, or 2.4 to 0.6 mm. The work is specifically directed at a partial reduction of iron ore, with the objective of reducing hematite to magnetite that could be concentrated by wet magnetic separation. Such procedure does not provide reducing conditions strong enough to reduce hematite to wustite and nickel oxide to metal and would therefore only provide up to 20% pre-reduction if applied to laterites.
A paper from Hirsch et al. presented at the first international conference on Circulating Fluidized Beds in Halifax, Nova Scotia, Canada in November 1985, generally describes the fields of application of circulating fluidized beds in metallurgy. It is merely suggested in the paper that circulating fluidized beds can be used for the pre-reduction of lateritic nickel ores.
There is therefore a great need to develop an efficient method for the pre-reduction of lateritic ores that could overcome the drawbacks listed above and provide an optimized method. Such method should allow the production of calcine containing the lowest possible concentration of carbon to prevent excessive reduction in the subsequent smelting step, resulting in off-specification product ferronickel.
In accordance with the present invention, there is now provided a method for the pre-reduction of an iron oxide-containing material such as nickel laterite ore, preferably fines, in a reactor with reducing gases generated in situ by the partial combustion of a reducing agent to produce a reactor calcine product with a low carbon content and a high degree of pre-reduction, the method comprising the steps of:
injecting an oxidizing gas into a chamber of the reactor, and feeding the iron oxide-containing material and a reducing agent;
maintaining a temperature inside the chamber sufficiently high to partially combust the reducing agent and provide a reducing atmosphere to convert Fe2O3 to FeO; and
recovering the reduced calcine product.
Preferably, the reducing agent has a particle size of from substantially 20 xcexcm to substantially 400 xcexcm. The oxidizing gas is preferably air, oxygen-enriched air, oxygen, CO2, steam, and mixtures thereof, air being the most preferred gas for obvious economic reasons.